[System] Added missing ctor to Socket for initializing dual-mode socket

 - Ref: https://bugzilla.xamarin.com/show_bug.cgi?id=20048
 - It's new one added in .NET 4.5
 - Absence of this ctor prevents KestrelHttpServer (web server for ASP.NET vNext: https://github.com/aspnet/KestrelHttpServer ) test code from working
 - I hesitated to put IPv4 only implementation, but whole dual stack (IPv4/v6) support would need bunch of works to be done. So, I pinned supported protocol of new ctor to IPv4